1. Field of the Invention
This invention relates to preparation of oxide layers on materials such as metals and semiconductors, and relates more particularly to catalytic oxidation of such materials, at temperatures near ambient, to form high quality ultrathin insulating materials on process intermediates useful in producing integrated circuit devices.
2. Description of the Prior Art
In recent years, there has been considerable interest in the room temperature oxidation of polycrystalline niobium because of its application as tunnel barrier material in superconductive Josephson junctions. The prime concern was to find direct evidence for the existence of suboxides which are thought to be the cause of large excess currents. X-ray photoelectron spectroscopy (XPS) studies showed that the oxidation of niobium is characterized by an initial fast oxygen sorption, then the formation of a thin layer of NbO and NbO.sub.2 followed at larger oxygen exposures by the formation of Nb.sub.2 O.sub.5 on the surface. The lower oxides of niobium exhibit inferior insulating characteristics as contrasted to Nb.sub.2 O.sub.5. Sanz and Hofmann determined the kinetics of Nb.sub.2 O.sub.5 formation, which under ultrahighvacuum (UHV) conditions in dry oxygen followed a logarithmic growth law, attaining a thickness of 0.2 nanometer after an oxygen exposure of 10.sup.4 L (1L=10.sup.-6 Torr.sec=1.32.times.10.sup.-4 Pa.multidot.sec). J. M. Sanz & S. Hofmann, J. of the Less-Common Metals, Volume 92, p. 317 (1983).
XPS and core electron energy loss spectroscopy studies of the surface oxidation of cerium suggest the initial formation of Ce.sub.2 O.sub.3 followed by a thin layer of CeO.sub.2 on the surface. Only trivalent oxide is formed upon oxidation with water vapor. See B. E. Koel et al., J. Electr. Spectr. Rel. Phen. Vol. 21, p. 31 (1980), and G. Strasser and F. P. Netzer, J. Vac. Sci. Tech., Section A, Vol. 2, page 826 April-May 1984.
There are a number of metals, including aluminum and niobium, which form intrinsic oxides when exposed to air at ambient temperatures. However, there is no known catalytic oxidation of solids prior to that of laboratory work resulting in this patent specification. Further X-ray photoemission spectroscopy studies (XPS) showed that the air oxidation of niobium could be suppressed by the use of thin aluminum, thin yttrium or magnesium, ytrrium or erbium overlayers.
Techniques for preparing insulating oxide layers generally required elevated temperatures, even though these temperatures were known to be undesirable because they exascerbated other problems such as spread of contaminants or diffusion of dopants beyond the optimum. Studies of oxides were carried out in a great number of laboratories because of the great interest in integrated circuits. These studies did not result in the knowledge that ambient temperature oxidation of meaningful insulation layers could be accomplished at all, much less provide the knowledge of actual techniques for carrying out any such oxidation, still less provide the knowledge of actual catalyst and method choices. Oxide layers required for semiconductor integrated circuits and other integrated circuits, continued to require elevated temperatures.
Certain catalysts, such as platinum, rhodium, palladium and cerium, but in particular platinum, have become common in the automobile industry for purifying exhaust gases. The exhaust catalytic reactions take place at elevated temperatures and involve gaseous components only. See, for example, U.S. Pat. No. 4,274,981, Suzuki et al., CATALYST FOR PURIFYING EXHAUST GAS AND THE PROCESS FOR MANUFACTURE THEREOF, June 23, 1981.
Certain rare earths and metals have known catalytic characteristics. Germanium, cerium, thorium, manganese, niobium, chromium, praseodymium, yttrium, zirconium, ruthenium, gallium, tin, indium, copper, lanthanum, tantalum and tungsten, plus others, have been listed in a context of oxidation promotion. See, for example, U.S. Pat. No. 4,001,317, Grasselli et al, PROCESS FOR THE OXIDATION OF OLEFINS USING CATALYSTS CONTAINING VARIOUS PROMOTER ELEMENTS, Jan. 4, 1977.
The prior art deals with gaseous and organic materials, not with process intermediates used in production of semiconductors, and not with oxidation of solids.